The invention relates to location management, and more specifically, to location management in personal communication services (PCS) for tracking the location of mobile subscribers between call arrivals.
IS-41and GSM protocols are defined for PCS intersystem operations to support PCS network management. A separate, out-of-band common control signaling (CCS) provides supervisory, addressing, call information provisioning functions in the telephone network. The signaling system No. 7(SS7) network is a CCS systems developed to satisfy the signaling requirement for call control and mobility management between the public switched telephone network (PSTN) and personal communication network (PCN).
The location management in the PCNs tracks the location of the mobile station (MS). The IS-41and GSM network deploy a two-tier system of mobility databases, Home Location Register (HLR) and Visitor Location Register (VLR), to store the location information of MSs. FIG. 1(a) depicts the location area hierarchy in GSM networks. A mobile service area is portioned into location/registration areas (LAs or RAs) 110. Every LA 110 substantially consists of a group of base transceiver stations (BTSs) 108 that communicate with the MSs over radio links. The geographic region served by a BTS 106 is also known as a cell in mobile communication systems. The BTSs 106 in an LA 110 are connected through a base station controller (BSC) 106 to a mobile switch center (MSC) 104. One or more MSCs 104 are connected to a VLR 102 which exchanges location information with the HLR 100.
FIG. 1(b) depicts the reference CCS network architecture with SS7 protocol. The signal messaging and exchanging among MSCs, VLRs, and HLRs are on top of the SS7 network which is in essence a two-level hierarchy corresponding to the location area hierarchy. The base stations within an RA 128 are connected via a wireline network to a service switching point (SSP) 126, i.e., an MSC, connected or collocated with a VLR. Different MSCs are connected to a local signal transfer point (LSTP) 124 via local A links 125, while all LSTPs in the same region are connected to a regional signal transfer point (RSTP) 122 via D links 123. The RSTP is also connected, via a remote A line 121, to a service control point (SCP) associated with an HLR.
The location management procedure in current PCNs involves location registration/update and terminal paging operations. When an MS moves from one LA to another, the MS performs the location registration/update to inform the network system. The network thus maintains the location information in the granularity of the location area. For call termination, however, the connection must be terminated at the cell where the target MS resides. To locate the MS, the terminal paging procedure pages all the cells in the LA. The described location management is based on an LA-based update and blanket paging strategy.
There exists a trade-off between the costs of location update and terminal paging operations. In general, location update operation includes transmitting signaling messages and information among network entities, and updating the records of the MS in the VLR and HLR. Although the network maintains the most recent location information whenever an MS makes a location movement, the LA-based update strategy consumes excessive network and database resources if the call rate to the MS is relatively low. In contrast, the terminal paging process must page a large coverage area, containing more cells, and thus the paging delay increases if the MS does not update its location frequently. Moreover, if the call rate to an MS is high, the cost of terminal paging is heavy. The LA-based update and blanket paging strategy is not cost-effective.
The location movement of an MS can be classified into three basic types: inter-LA, inter-MSC and inter-VLR movements. The inter-LA movement mainly makes the exchange of signaling messages within an LSTP region. Comparatively, the inter-MSC and inter-VLR movements have additional exchange of signaling messages among the target VLR, HLR and previous VLR in the RSTP region. In general, the cost of exchanging messages in the LSTP is lower than that in the RSTP.